1. Field of the Invention
This invention relates to a .beta.-silicon nitride sintered body which is a so-called fine ceramic material high in fracture toughness and strengths and serves as a variety of mechanical and structural parts extensively used in the fields of automotive vehicle, machine parts, chemical apparatus and aerospace instrument, and to a method of producing such a .beta.-silicon nitride sintered body by using inexpensive .beta.-silicon nitride as the raw material.
2. Description of the Prior Art
A sintered body formed of silicon nitride based material is chemically stable and has high mechanical strengths at ordinary and high temperatures. Accordingly, the sintered body is suitable for sliding parts such as a bearing and engine parts such as a rotor of a turbocharger.
It has been hitherto a common opinion in the field of ceramics that a raw material having .alpha.-silicon nitride as a main component is necessary to obtain a high strength silicon nitride sintered body. In general, commercially available powder containing 90% by weight or more of .alpha.-silicon nitride has been used as the raw material for the sintered body. Such a raw material powder containing .alpha.-silicon nitride has been used to produce a .beta.-silicon nitride sintered body for the reasons set forth below.
(a) .alpha.-silicon nitride is usually in the form of fine powder and therefore high in sinterability.
(b) .alpha.-silicon nitride makes its phase transition from .alpha.-phase to .beta.-phase during sintering and therefore columnar crystal therein develops so as Co raise its strength and toughness. However, the above discussed conventional silicon nitride requires to control the content of .alpha.-phase in powder of the raw material. This unavoidably complicates a preparation process of the raw material thereby raising cost of the raw material.
On the other hand, a silicon nitride powder of which main component is .beta.-silicon nitride has been known as being used a raw material for refractory products. Additionally, a sintered body from .beta.-silicon nitride is known and disclosed, for example, in J. Am. Ceram. Soc. Vol. 57, page 25, 1974 and in Japanese Patent Provisional Publication No. 58-151371. However, .beta.-silicon nitride is relatively coarse or large in particle size, and additionally is low in content of .alpha.-silicon nitride. Accordingly, with such silicon nitride powder, no columnar texture cannot be formed in the sintered body so as not to provide a high strength to the sintered body. As a result, the silicon nitride powder including .beta.-silicon nitride as the main component has not been used as the raw material for producing a high strength .beta.-silicon nitride sintered body.
In view of the above, one of the inventors of this application has previously developed and proposed a gas pressure sintering method in which sintering becomes possible at a high temperature under a high nitrogen gas pressure, as disclosed in Japanese Patent No. 1,247,183. Additionally, it has been demonstrated that according to the gas pressure sintering method, sintering a high density can be made even upon using .beta.-silicon nitride powder which has been conventionally deemed to be low in sinterability, as disclosed in J. Materials Science, Vol. 11, pages 1103 to 1107, 1976 and in Japanese Patent Publication No. 58-151371. Furthermore, one of the inventors has proposed that a high strength .beta.-silicon nitride can be obtained by controlling the particle size distribution of high purity .beta.-silicon nitride powder, as disclosed in Japanese Patent Provisional Publication No. 2-255573.
However, difficulties have been encountered even in the above discussed techniques developed by one of the inventors, in which columnar structure cannot be effectively developed in the sintered body so that the sintered body is not sufficient in strength, and a precision control of particle size distribution is required for the high purity .beta.-silicon nitride powder as the starting material.